Generally, fluid control systems for power shovels are adapted to control the running of the machine body and the pivotal movement and swiveling of the boom, arm and bucket by supplying a pressure fluid from two main pumps to actuators, such as a pair of running motors, boom cylinder, arm cylinder and swivel motor, via opposite running control valves and working device control valves for the boom, arm, bucket and swivel body. These control valves are divided into two groups so as to be easily operable at the same time.
Since power shovels are used for work on large and small scales as working machines in place of manual work, they are operated in a wider variety of modes. The operations of the power shovel include, for example, running only, a single movement of the boom, arm, bucket or swivel body with the machine body in a fixed position, and horizontal excavation or work on slopes wherein the forward end of the shovel is moved along a horizontal surface or the slope through two or more simultaneous operations. In any of these cases, it is required to operate the power shovel efficiently. Further with the power shovel, the working device, especially the boom and the arm, must be moved powerfully and rapidly, so that when the boom or the arm is to be moved, the fluid discharged from the two main pumps needs to be supplied as a confluent flow to the actuator concerned. Further when the working device is to be operated during running, the machine body must be made to run straight.
To fulfill these requirements, Unexamined Japanese Patent Publication SHO 62-107124 discloses a fluid control system, which comprises a first boom control valve for supplying the pressure fluid from one of two main pumps to the boom cylinder, and a second boom control valve for supplying the pressure fluid from the other main pump, as joined with the fluid from the first-mentioned pump, to the boom cylinder. Similarly, the system includes a first arm control valve and a second arm control valve for the arm cylinder. One of two running control valves, the first boom control valve, a bucket control valve and the second arm control valve form a control valve group, and the other running control valve, the first arm control valve and the second boom control valve form another control valve group. Further to enable the machine body to run straight, a flow-dividing selector valve (pilot directional control valve) is provided between the two main pumps and the two control valve groups.
When the power shovel having the conventional fluid control system described is caused to run only, the pressure fluid discharged from the main pumps is supplied to the running control valves independently of each other by the flow-dividing selector valve. When one of the working device actuators is to be operated simultaneously with running, the pressure fluid is supplied from one of the main pumps to the opposite running control valves, and the pressure fluid from the other main pump is supplied to the control valve for the actuator, whereby the power shovel is maintained in straight travel. When the boom cylinder is to be moved, the first and second two boom control valves are operated, such that the pressure fluid flows from the two main pumps are passed through the two boom control valves and supplied as joined together to the boom cylinder. Thus, the boom cylinder, i.e., the boom, is operated powerfully and rapidly. The arm is moved also in the same manner.
With the conventional fluid control system, however, the first and second two control valves are necessary for the boom cylinder, as well as for the arm cylinder. Each of these control valves must be a three-position directional control valve which is operable with high accuracy for flow control and which is therefore complex in construction and costly. Further when the pressure fluid portions from the two main pumps are to be joined together and supplied to the boom cylinder or arm cylinder, the pressure fluid portion flowing from one of the main pumps into the first control valve and the fluid portion flowing from the other main pump into the second control valve are joined together by piping provided externally of the control valve groups and then supplied to the cylinder. Accordingly, the external piping is complex in construction and is likely to permit leakage of the fluid at the piping joint. Further with the fluid control system described, the first boom control valve and the second arm control valves are connected in parallel with each other, and the first arm control valve and the second boom control valves are connected in parallel with each other, so that in the case where the boom cylinder and the arm cylinder are operated at the same time, the control system is influenced by the weight of the working device, e.g., of the boom, arm, etc. or encounters variations in excavation resistance. The pressure fluid is therefore likely to flow into the actuator of smaller load, with the result that the actuator of greater load operates less effectively.
Further with the power shovel, the load on the working device actuators is likely to vary greatly with the work to be performed. Accordingly, it is required to operate the actuator rapidly when the load is small. A fluid control system so adapted is known which includes a working device control valve having a spool and in which when the load is small, the spool stroke is restricted to a position, and at this position, the fluid discharged from the actuator is caused to flow into the actuator again.
The known control system is so designed that the stroke of the control valve spool is restricted with use of a pilot pressure delivered from the secondary side of a pilot valve for changing over the control valve. However, the pilot pressure varies with the amount of control of the pilot valve and is unstable, consequently making the spool stroke restricting position unstable and failing to permit the fluid to flow into the actuator with stability to cause hunting of the actuator.
An object of the present invention is to overcome the foregoing problems of the prior art and to provide a fluid control system for operating a power shovel efficiently, the control system being adapted to automatically divide or join within a control valve group a pressure fluid from two main pumps by changing over the desired control valve so as to supply the fluid to an actuator with use of simplified external piping and with a reduced likelihood of the fluid leaking, the control system further enabling the machine to run straight effectively, permitting a working device actuator, such as a boom cylinder or arm cylinder, to operate singly with an increased speed, and also permitting at least two working device actuators, such as the boom cylinder and arm cylinder, to operate simultaneously and properly at a definite speed ratio.
Another object of the present invention is to provide a fluid control system for a power shovel which system is adapted to operate a working device actuator, such as arm cylinder, to be subjected to varying loads by causing a pressure fluid discharged from the actuator to flow into the actuator again for reuse when the load is small to operate the actuator at an increased speed and to reuse the fluid with good stability while preventing the actuator from hunting, the control system further being adapted to operate the actuator powerfully with an increased pressure without reusing the fluid when the actuator is heavily loaded.